If you get error "Pages Public Content Access requires either app secret proof or an app token." - it means that you have access token of the Facebook Feed Pro version prior to 2.0.0.

In this case you need to follow these steps:

  1. Go and edit the widget that throws that error
  2. Click "Disconnect" button and disconnect the app
  3. Click "Connect to Facebook" and provide necessary permissions
  4. Click "Save" and save the widget

Earlier (v.1.6.2 and lower) it was possible to display any public page (with any Facebook account, no matter if it is admin or not), but this year Facebook disables this option.

If you have v.1.6.2 - you can still use it with any public page, but any time Facebook may close this option as it was announced last year.